Wind energy storage technologies - Wind Power Generation and Wind Turbine Design - page 685

Battery technology such as the LA battery is the main competitor for FES. These
have similar characteristics to FES devices, and usually cost 33% less [3]. How-
ever, as mentioned previously (see Section 3.7.1), FES have a longer life span,
require lower maintenance, have a faster charge/discharge, take up less space and
have fewer environmental risks [2].

4.6.3 Disadvantages of FES
As fl ywheels are optimised for power or storage capacities, the needs of one appli-
cation can often make the design poorly suited for the other. Consequently, low
speed fl ywheels may be able to provide high power capacities but only for very
short time period, and high speed fl ywheels the opposite. Also, as fl ywheels are
kept in a vacuum during operation, it is diffi cult to transfer heat out of the system,
so a cooling system is usually integrated with the FES device. Finally, FES devices
also suffer from the idling losses: when fl ywheels are spinning on standby, energy
is lost due to external forces such as friction or magnetic forces. As a result, fl y-
wheels need to be pushed to maintain its speed. However, these idling losses are
usually less than 2%.

4.6.4 Future of FES
Low maintenance costs and the ability to survive in harsh conditions are the core
strengths for the future of fl ywheels. Flywheels currently represent 20% of the
$1-billion energy storage market for UPS. Due to its size and cycling capabilities,
FES could establish even more within this market if consumers see beyond the
larger initial investment. As fl ywheels require a preference between optimization
of power or storage capacity, it is unlikely to be considered a viable option as
a sole storage provider for power generation applications. Therefore, FES needs
to extend into applications such as regenerative energy and frequency regulation
where it is not currently fashionable if it is to have a future [3].

4.7 Supercapacitor energy storage

Capacitors consist of two parallel plates that are separated by a dielectric insulator
(see Fig. 14). The plates hold opposite charges which induce an electric fi eld, in
which energy can be stored. The energy within a capacitor is given by:

CV

( 4 )

E =

2

where E is the energy stored within the capacitor (in J), V the voltage applied, and
C is the capacitance found from [1]:

A ee

(5 )

r0

C =

d

where A is the area of the parallel plates, d the distance between the two plates,
e r the relative permittivity or dielectric constant, and e 0 is the permittivity of free